Bill Gates says there would be consequences if the UK cut its aid pledge: ‘Ambitious things going on with malaria, agriculture and reproductive health simply would not get done’. Photograph: Linda Nylind/the Guardian

Bill Gates warned Theresa May that should the Conservatives go ahead and abandon the UK’s overseas aid spending pledge it would reduce the country’s influence in the world and mean more lives lost in Africa.

Speaking to the Guardian on the second day of the general election campaign, the billionaire philanthropist spelled out the potential consequences of dropping the existing target of spending 0.7% of GDP on aid, hours after the prime minster refused to recommit to the target.

Downing Street sources also refused to say whether the target would be included in the party’s election manifesto when questioned. Labour believes May wants to cut the £12bn budget and abandon the UK’s commitment to the developing world.

Gates said: “The big aid givers now are the US, Britain and Germany – those are the three biggest and if those three back off, a lot of the ambitious things going on with malaria, agriculture and reproductive health simply would not get done.”

But the Microsoft founder said that the leadership role taken by the UK could determine whether ambitious efforts to eradicate malaria in Africa were launched.

He added: “Malaria has always been the disease we really want to take on and the UK has always in terms of research capacity and aid been a leader. In terms of where the aid ambition gets set, the UK can be a huge leader in driving that malaria eradication, or the world may have to back off and not get started on that.”